Fast ions as a tool for plasma control and enhanced performance on JET

High-energy alpha particles will be the main source of plasma heating in ITER and future fusion reactors. Extrapolating the performance of plasmas with dominant alpha particle heating is not straightforward [1]. Indeed, MeV-range alphas can simultaneously destabilize Alfvén eigenmodes (AEs), impact the sawtooth dynamics, modify plasma equilibrium etc., altogether defining the resulting plasma performance [2].

We discuss recent findings from a series of dedicated experiments at JET, aiming to generate a large population of MeV-range ions and maximize the fast-ion pressure in the plasma core [3]. A number of new fast-ion physics phenomena have been revealed in these studies. First, we report on unexpectedly high T_i and improved thermal ion confinement in JET plasmas with electron heating from MeV ions [4, 5]. Second, we discuss a novel technique for fast-ion current drive, leading to sustained plasmas with an inverted q-profile and a unique observation of high frequency reversed-shear AEs at JET [6]. These modes, earlier observed in JT-60U plasmas heated with N-NBI, are highly relevant for ITER, but have been scarcely documented so far. We also discuss progress on synergistic effects that arise from the simultaneous occurrence of different fast-ion phenomena. In particular, we report on novel measurements and characterization of AEs with toroidal mode numbers n<0 and n=0, observed in D-³He plasmas with fusion-born alpha particles [7]. A new mechanism leading to the generation of a fast-ion population with a bump-on-tail ∂f/∂E<0 in the presence of monster sawteeth was identified recently [8].

These findings formed a basis for the follow-up fast-ion studies in the recent DTE2 campaign. We conclude with discussing the impact of AEs driven by ICRF-generated fast ions on the performance of D-T plasmas at JET and present evidence of non-linear mode coupling in these plasmas.